The present invention relates in general to detecting received signal quality in a communications receiver, and more specifically to full-complex FM detection in an FM receiver to obtain a quality indicator signal as a measure of noise present in a received FM broadcast signal.
It is well known that in all forms of electronic wire or wireless communications, many types of noise are inadvertently added to a signal when it is transmitted through a transmission channel. Efforts are usually made at the receiving end to detect the presence of excessive additive noise and to take corrective actions or to modify signal reception characteristics as necessary to reduce the impact of the noise.
In an FM radio broadcast system, for example, background emissions, multipath reflections, and off-channel FM broadcasts can all cause noise in a particular FM channel. It is known to detect noisy conditions in an FM channel by measuring high frequency noise energy in the demodulated signal. Based on this measurement, corrective actions are then taken such as reducing stereo separation (i.e., increasing blend) toward monophonic reproduction. This reduces the noise impact because the higher frequency stereo information is more strongly affected by noise. Other corrective actions include narrowing the receiver bandwidth to remove proportionally more noise than original signal, or switching to another antenna in a diversity system which includes spaced antennas.
Examining higher frequencies above the main audio range of a received broadcast signal gives an indication of when noise can be expected to be present at lower frequencies which affects reproduction of the FM signal. However, high frequency noise energy can be an imperfect indicator of noise in the audio range for certain types of noise (either under-estimating or over-estimating the lower frequency noise affecting the FM signal). Thus, a method for detecting noise in the actual frequency range of interest is desirable.